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The agencies have different mandates -- the EPA holds sway over air and water pollution, while Interior administers the nation's vast federal land holdings as well as the Endangered Species Act -- but both deal with some of the country's most pressing environmental concerns, such as climate change. And over the past eight years, many career employees and rank-and-file scientists have clashed with Bush appointees over a number of those of issues, including whether the federal government should allow California to regulate tailpipe emissions from automobiles and how best to prevent imperiled species from disappearing altogether.

In June 2007, Obama told reporters in Reno, Nev., that he would not hesitate to reverse many of the environmental policies Bush has enacted by executive order.